Cyclosporin A (CyA) strongly inhibited production of interleukin 3 (IL3) by normal mouse spleen cells and by the T lymphoma LBRM-33 clone 1A5 induced by alloantigens and by the lectins concanavalin A or phytohemagglutinin. At the same concentrations at which CyA exerted this suppressor activity, it did not affect the growth of the Ea3.15 pre-B cell clone supported by recombinant mouse IL3 (the read-out for IL3 activity used in this study) nor did it show significant inhibitory effect on the spontaneous secretion of IL3 by WEHI-3 cells and by the LD1 T helper line. Taken together, these observations indicate that CyA does not interfere with translation, synthesis or secretion of IL3. CyA most likely interferes with the process by which antigens and lectins induce transcription of genes coding for lymphokines.